The present invention relates to a pipe bending machine with a travelling carriage having a clamping sleeve, a bending template with a clamping jaw and an adjacent slide roll, and a bending mandrel arranged in a pipe to be bent in the region close to the bending template and having a mandrel rod at its head end.
During cold bending of pipes, a mandrel rod is used in many cases so that during bending the round inner crosssection of the pipe is obtained or continuously maintained. Also for bending of two pipes insertable into one another, for example such as disclosed in the DE-OS No. 2,732,046 of the same inventor, bending mandrels are used.
Bending mandrels which act in the region of the bending template and the slide rail are mounted on a mandrel rod which has a smaller cross-section as the bending mandrel. The mandrel rod is connected with a piston of a cylinder-piston unit which displaces the mandrel rod with the mandrel for bending and after bending withdraws the same.
The pipe to be bent on a pipe bending machine is displaced for loading the pipe bending machine via the mandrel and the mandrel rod, and it is not always guaranteed that it is rectilinear. For example, individual pipes can be slightly curved which in many cases results in damaging during the transportation.
Many pipes are produced in such a way that they are bent from a metal sheet strip to a pipe and then welded in a longitudinal direction. The welding seam is in many cases dimensioned so that no subsequent treatment is needed to remove inwardly or outwardly projecting material of the welding seam. It is however not excluded that the parts of the welding seam extend inwardly and thereby lead to a narrowing of the inner pipe cross-section.
Pipes which are not straight or pipes in which a part of the welding seam projects inwardly make more difficult the passage of the pipe over the bending mandrel. Since this passage is performed by a machine, it often happens that the inner cross-section of the pipe reduced by bending or by a welding seam not only upsets the mandrel rod during passage of the abutments of the mandrel on the inwardly projecting pipe wall resulting in the reduction of the pipe cross-section, but instead in many cases it is so bent by upsetting that it becomes unuseable and must be exchanged by another mandrel rod. Since during bending of the mandrel rod in many cases the pipe cannot be displaced over the mandrel rod while the pipe feeding still takes place, it is not included that in addition to the bending of the mandrel rod further damages can take place.
In connection with this it is also possible that the pipe provided with the reduced pipe cross-section displaces over the mandrel of the mandrel rod, but leads to such damages of the mandrel rod pipe by bending that first during bending of the pipe on the bending template it can be recognized, for example by the way in which after bending the withdrawal or prior to bending the displacement of the mandrel via the mandrel rod becomes difficult or completely impossible.
When a pipe is bent over the bending templates, the subsequent pipe bends are in many cases produced in another bending plane. For attaining this, the pipe is turned prior to the bending. During this rotation the bending mandrel remains in the pipe. When for the above-mentioned reasons the pipe cross-section is reduced at certain locations, the reduced pipe cross-section can also lead to clamping of the mandrel on the inner wall of the pipe with the result that during rotation of the mandrel because of the clamping sleeve of the travelling carriage, the mandrel rod is subjected to the action of torsion which extends beyond the plastic region of the mandrel rod and can lead to a permanent deformation of the mandrel rod and also to a torsion breakage. A damage to the mandrel or to the mandrel rod or resulting subsequent damages in parts of the pipe bending machine, for example in the mandrel withdrawal means and the like, are especially serious for pipe bending machines which operate automatically, for example, with automatic loading for bending and withdrawal of the bent pipes from the pipe bending machine in a fully-automatic manner, such as disclosed in the DE-OS No. 2,832,980 of the same inventor. In such bending machines with automatic loading and automatic withdrawal even small deformations of the bending mandrel over its mandrel rod exceeding a certain tolerance can lead to a strong hindering of the programmed course.